The western border of the Archaean province of the Baltic shield: evidence from northern Sweden

Restricted areas of acid to intermediate gneisses in northernmost Sweden are known to be Archaean and several other gneissic granodioritic to tonalitic massifs have been suggested to be of this age. To delimit the western border of the Archaean province of the Baltic Shield, and to characterize the Swedish Archaean, we have studied the 2.83 Ga old Soppero gneiss and 8 of the proposed Archaean massifs in northern Sweden by Sm-Nd isotopic analyses, U-Pb zircon dating (in two cases), and geochemistry. Of the Archaean gneisses proposed thus far, only those from the Soppero and Kukkola areas have Archaean ages (2.83 and 2.67 Ga, respectively). These gneisses have geochemical characteristics such as LREE-enrichment, low U-contents, and low K/Na-ratios, which are typical of Archaean TTG (tonalite-trondhjemite-granodiorite) rocks worldwide. The REE results indicate that the Soppero and the Kukkola gneisses were generated by partial melting of basic rocks, presumably amphibolites. According to the Sm-Nd isotopic data for the Soppero gneiss, this process occurred only a short time after initial segregation from the mantle (εNd values between +0.9 and +3.5). The Kukkola gneiss, however, has longer crustal residence time as indicated by negative εNd values (-1.0 and -2.5) at the U-Pb zircon age of 2.67 Ga. Evidence in support of an important event at that time is given by secondary overgrowth on zircon cores in the 2.83 Ga old Soppero gneiss. The Sm-Nd isotope results show that the Proterozoic granitoids in northern Sweden have a decreasing involvement of Archaean source material with increasing distance from the Archaean areas. Before the Proterozoic reworking, the rather small Archaean areas in northern Sweden presumably belonged to a large continuous Archaean craton. The western palaeoboundary of this craton in Sweden probably follows approximately a line extrapolated from the western limit of the Archaean of central Finland, passing west of Kukkola, and then continues to the Lofoten area in northern Norway.